Polyacetal (also called polyoxymethylene, POM) resins exhibit excellent mechanical and physical properties, such as tensile strength, stiffness, as well as fatigue resistance, self-lubrication, chemical resistance, moldability and the like. These resins are extensively used as an engineering plastic material in various applications that include automotive and domestic appliances, in construction, in machinery, and in the tool, electronics and consumer goods industries. In addition to their excellent mechanical and physical properties, polyacetal resins are known to exhibit good self-lubricating properties and wear resistance and have been used for applications having moving parts that contact other parts.
Polyacetal resins have been variously modified to provide add-on properties. Examples of resin modification include the addition of fillers to provide an almost unlimited range of compositions to fit a wide range of engineering applications. Nonetheless, additives or fillers in polyacetal compositions often have adverse effects on the composition's processability and in particular, on moldability, extrudability, mechanical properties and thermal stability.
Degradation and low thermal stability are common drawbacks of polyacetal resins, which are inherently unstable in the presence of air at high temperature and decompose to generate formaldehyde during processing or shaping of articles. This in turn is chemically active and readily oxidizable to formic acid, which can decompose polymeric chains and adversely affects the heat resistance of the polyacetal resin.
U.S. Pat. App. Pub. No. US 2006/0111473 discloses a high-density polyacetal composition that comprises at least one surface-coated mineral and which are said to have improved thermal stability. The process of making such high-density polyacetal resins involves at least two steps: first, surface-coating the mineral with an appropriate coating agent; and, then melt-mixing the coated mineral with the resin. The complexity of the current manufacturing processes renders the production of these polyacetal resins time consuming and hampers a high productivity of the process.
There remains a need for a simpler and more efficient method of incorporating mineral fillers into a polyacetal composition, which avoids the decomposition of the polyacetal composition, and which results in polyacetal compositions that exhibit both maintained or improved thermal resistance and maintained or improved mechanical properties.